Inversion by "only" in a passive sentence

Solution 1:

Passive does not invert its auxiliary be; it just inserts it and turns the verb to past participle.
Promoting the direct object to subject is not inversion, which is simple re-ordering of words.

The "must be turned into" part of your proposed transformation is wrong, with preposed only.
Inversion is not required with preposed adverbs, although it is optionally grammatical
with preposed negative adverbs of place, time, or circumstance (i.e, ones that negate the clause).

• At no time was the temperature changed.
• Nowhere may the temperature be changed from the thermostat settings.
• Under no circumstances is the temperature to be changed.

Only, however, is not one of those. If you try this with different adverbs, you get ungrammaticality:

• *With no instrument was the salami sliced by George.
• *In no particular way did she say the sentence.
• *Only is the temperature changed in the daytime.

Solution 2:

1. Only the temperature is changed.

2. *Only is the temperature is changed. (wrong)

Inversion of the type described by the Original Poster does not occur in main clauses when only is brought to the front of the main clause. This can be seen in example (2).

However, moving only to the front of a sentence can, and sometimes must, trigger subject auxiliary inversion in main clauses under the right conditions. Here are some examples:

• Only if we've received the papers can we release the prisoner.
• *Only if we've received the papers we can release the prisoner. (wrong)
• Only after the concert did I notice Pavarotti in the back row.
• *Only after the concert I saw Pavarotti in the back row. (wrong)
• Only in extreme circumstances did they steal.
• *Only in extreme circumstances they stole. (wrong)
• Only in the houses of parliament will you find this many cads.
• *Only in the houses of parliament you will find this many cads. (wrong)

These sentences have certain features in common. They all have adjuncts (read adverbials) in the form of Preposition Phrases. These Preposition Phrases all precede the main clauses. The head preposition in each case is being modified by only.

Only must have the sense of not except in such examples. Notice as well that if the adjunct is parenthetical in nature, then the inversion is not necessary. In the examples above, commas have not been used implying that the sentences are presented as one long integrated tune. In speech, if there is a pause, and the two phrases are said with discrete intonation patterns, then the non-inverted examples will usually be deemed grammatical:

• Only if we've received the papers mind - we can release the prisoner.
• Only in extreme circumstances - they stole.

Notice as well in the main examples that although only modifies the subordinate phrase, the inversion occurs in the main clause. The subject auxiliary inversion does not occur in the subordinate clauses.

One last point of note: this phenomenon is quite different from that of sentences with preposed negative adverbs. In sentences such as:

• Never have I seen such a debacle.

... the negative adverb fronts a main clause. Also, the adverb is not modifying the word it appears in front of. Importantly, the subject auxiliary inversion occurs in the same clause as the adverb. In contrast, in the examples with only, this adverb does not occur in front of the main clause. Secondly, the adverb modifies the head preposition in the Preposition Phrase. Lastly, only occurs in the same phrase as the subordinate clause, but the subject auxiliary inversion occurs in the main clause.

This also shows that inversion caused by only has nothing to do with sentences featuring not only ... but also. The not only ... but also construction is clearly a case of negative adverb preposing. It is the adverb not which causes the inversion in such sentences. This is easily demonstrated by the fact that the subject auxiliary inversion happens in the same clause that the adverb not occurs with:

• Not only did he laugh, he cackled.

Solution 3:

First of all, we ought to write a passive voice sentence with a common structure, where the word 'ONLY' is not at the beginning of the sentence, for instance;

The temperature is changed only if there is a risk of a spill.

Now, we must know that when we use the word 'ONLY' at the beginning of a clause or sentence, there must be an INVERSION (From; Subject-Verb/Auxiliary to Auxiliary-Subject).

Only if there is a risk of a spill is the temperature changed.

Usually, the word only is followed by these words;

Only after... (After what?)... (then the Inversion)... Only by... (by what?... (then the inversion)... Only if... (if what?)... (then the inversion)... Only in this way (then the inversion)... Only Then (then the inversion)... Only When... (when?)... (then the inversion)...

Some passive voice examples in sentences are;

Only after having read her will were they allowed to claim their inheritance. Only by winning the championship again will the world record be broken. Only in this way are endangered animals ** ** from poachers. in

IMPORTANT: If only is followed by the subject at the beginning of the sentence, there is no inversion:

Only you can understand. (no one else can understand.)

For further information visit https://www.grammaring.com/only-after-only-if-only-in-this-way-etc-not-until